Radiotelephone receiving system



June l, 1954 w. R. YOUNG, JR

RADIOTELEPHONE RECEIVING SYSTEM 4 Sheets-Sheet 1 Original Filed May 20, 1949 June l, 1954 w. R. YOUNG, JR

RADIQTELEPHONE RECEIVING SYSTEM 4 Sheets-Sheet 2 original Filed May 2o, 1949 v. M RG. m W W f N A w of vm WV. R W v, B NW QQ NN June l, 1954 w R, YOUNG, JR v 2,680,194-,

RADIOTELEPHONE RECEIVING SYSTEM Original Filed May 20, 1949 4 Sheets-Sheet 3 RATIO IN DEC/EELS SIGNAL NOISE v1/Emo@ W R. YOUNG, JR.

A T TOR/VEV June 1, 1954 w. R. YOUNG, JR

RADIOTELEPHONE RECEIVING SYSTEM Original Filed May 20, 1949 4 Sheets-Sheet 4 Umgo EN To@ R. YOUNG, JR.

mw i w. Vm

ATTORNEY 448, filed Niay 20,1949, V.N0. 2,636,982 0f April 28,

bined, the phase vrents may be such as to undesirable eiiect.

improve the means for ,pany the received Patented June l, 1954 William R. Young,

Bell Telephone Labor Jr., Summit, N. J.,.assgnor to atories,IncorporatetLNew York, N. Y., a corporation of 'New York 'Grigninal application May 20, 1949, Serial No.

94,448, now Patent No. 27, 1953. Divided and t 2,636,982, dated April his application August 11, 1951, Serial No. 241,444

(Cl. Z50- 27) 4 Claims.

1 This is a division .of application Serial No. 94,-

now United States'Patent 1953, for'Radiotelephone Receiving System Employing a Plurality of Receivers.

This invention relates to electric signaling systems and more particularly to methods and means for selecting an individual message signal Wave from a plurality of such Waves.

It is frequently desirable to receive at a number Aoi geographically separated locations the Same .message signals, andto select from the received signals onlythat train of received signals which,

.ata-ny instant, provides the best communication channel. Multiple reception of the same message signals is frequently employed in mobile .radio communication systems and in systems in which the signals are transmitted through a variable transmission medium, such that signals received at one location may be subjected to considerably more attenuation thanare signals received at a diiierent location.

In such .communication systems, it isfgenerally believed to be desirable to select, at any instant, only one such train of received message signals, since if two or more such signal trains are comrelations of the combined curproduce a detrimental or Furthermore, such signal Wave energies are usually accompanied to a greater or lesserdegree by noise signal currents. Where such signal currents are directly combined, a more desirable train of signal waves may be impairedby the noise energies that accompany another andless desirable train of signal waves With which it is combined.

It is accordingly one object of this invention to selecting from a plurality of received message signals the train of message signals which., at any instant, is judged Yto be the most desirable.

-It is also an object of this invention to provide, in. a radio communication system, a means for receiving the message signal Waves in which the eifeet .of interfering noise currents is minimized, and in which the reliability of reception is increased.

It is a further object of this invention to pro- .vide means for continuously determining the amount of interfering-noise currents that accommessage signal waves, andto choose between the received message signals in .accordance with this determination.

In accordance withthe present invention, the

.same .transmittedmessage signal wave may be f by determining received at a number of geographically separated points, and the noise energy that accompanies each train of the received vsignal waves iscontinuously determined. At eachreceiver location there is derived a unidirectional voltage, 'the amplitude of which is indicative of the level of the interfering noise Waves that areV received with the message signals at that location. These voltage indications are transmitted to a centrally located selection point. At this selection point a means is Yprovided. for continuously comparing the noise-indicating amplitudes of 'these received unidirectional voltage signals, and for connecting to a utilization circuit the output oi the message signal receiver which produces the most desirable unidirectional signal. Means are also provided at this centrally located selection point for canceling the effect ci any induced longitudinal currents that may be added to these derived signals in the transmission paths between the receiver locations andthe selection point.

'It is a feature of this invention that the continuous determination of the noise energies that accompany the received message signals is made the energy level of the noise currents in the "frequency spectrum immediately above and beneath, but not within, the signal frequencyspectrum.

It is also a feature of this invention that this noise `energy determination made in such a manner that the dierent types of noise interference are Weighted in much the same manner as their interference effects are noted by the human ear.

1t is a further feature of this invention that the relative desirability of each received train of message signal waves is indicated by a unidirectional voltage signal, derived or otherwise generated electrical quantity or factor, for example, the amplitude of which .changes with variation in the noise energy that accompanies the signal wave.

Still another feature of the invention is that the eiects of longitudinal .noise currents that may be induced in the connections over which such merit indicating signals are transmitted, between the points of reception of the noise energies and the centrally located selection station, are nullied.

The manner in which the foregoing objects and features of theinvention are realizedmay be best understood from the following description when considered in conjunction with the drawing in which:

Fig. 1 is a block schematic diagram ofaradio receiver selection system in accordance with the invention when arranged to choose between the signal outputs of a plurality of receivers of the same message signal Wave;

Fig. 2 is a schematic diagram illustrating primarily one of the noise detecting and evaluating units that are incorporated in a receiver selection system in accordance with the invention;

Fig. 3 is a schematic circuit diagram illustrating primarily one of the longitudinal balancing units that may be used in a selecting system in accordance with the invention for counteracting the effect of induced longitudinal noise currents on the merit indicating signal;

Fig. 4 is a schematic circuit diagram illustrating primarily a signal comparison and selecting circuit in accordance with the invention, for comparing the noise or merit indicating signals from the various message signal receivers, and for connecting to a utilization circuit the receiver output that is associated with the most desirable noise indicating signal;

Fig. 5 is an explanatory graph to which reference is made in the circuit description; and

Fig. 6 shows how Figs. 2, 3 and 4 may be arranged to disclose the complete circuit of one branch of a selecting system which is arranged in accordance with the invention.

General description The invention will now be generally described as being incorporated in a system for selecting the most desirable one of four trains of the same message signal waves. It will be understood that, although the invention is described as being incorporated in a system for selecting the output of one of four message receivers, this number does not constitute a limitation on the invention, since systems embodying the invention may involve as few as two or be expanded to choose from .among any reasonable number of such message receivers. Referring to Fig. 1, the same message signal waves are received on the geographically separated antennas IG, I0', i, and 10". Each of these antennas is connected to a radio receiver i2, l2', I2, and i2, respectively, which receivers are substantially alike and may be adapted for the reception of either amplitudemodulated, phase-modulated, or frequency-modulated signal waves. Each receiver is preferably arranged such that the volume of its audio frequency output signal is substantially independent of the level of its received radio frequency signal waves, if the energy level of these waves exceeds a predetermined minimum value; but is determined principally by the intensity of the modulating signal, or level of modulation at the transmitter. Under these circumstances, the level of the audio frequency message signal wages is substantially the same in the output circuit of each receiver I2, I2', I2, or l'", at any instan if the received radio irequency signals exceed this minimum level. Noise interference energy that may be received by any one of the antennas lil, I0', etc. will be added to or included in the received message signal waves, and such interference energy will be more prominent in the receiver output product as its ratio to the audio frequency message signal energy in the receiver output is increased. The audio frequency output of each receiver I2, I2', etc. is divided, and one portion of this energy from each receiver is transmitted over connections I, Id', ctc. to a control terminal or selecting point, where it may be connected by a receiver selector I6 to a utilization circuit I8. The

second portion of this output energy is supplied to a respective noise detector 20, 26', etc., which is preferably situated at the same location as its radio receiver I2, I2', etc. Noice detector 20 operates on this portion of the output energy to extract from it only the noise energies that exist in the frequency spectrum immediately above and below. but not Within, the message signal band. It has been determined that, in general, the amount of noise in the voice frequency band of interest is closely related to the amount of noise which occurs above and below that band. A measure of these latter quantities is therefore an indirect but satisfactory measure of noise in the message signal band of interest. This extracted energy is then weighted in a manner which will presently be described before it is used to derive a unidirectional signal, the amplitude of which is indicative of the weighted amount of noise encountered in the previously mentioned portions of the frequency spectrum. The ampli tude of this unidirectional signal is related to the amount of extracted noise energy, and in this described embodiment, it varies in inverse relation to the amount of this noise energy. 1t should be appreciated that this inverse relation between the amount of extracted noise and the amplitude of the derived signal need not necessarily be foliowed, and it should not be construed to be a limitation upon the invention. These unidirectional merit indicating voltage signals from the noise detectors 2i), 20', etc. are transmitted to the control terminal, or selection point, over connections E2, 22', etc. For general descriptive purposes, these connections 22, 22', etc. are shown as being separate from connections ill, ill', etc. However, as will be later described in connection with Fig. 2, the paired connections III, 22 or I4', 22', etc. may actually consist of a conductor pair.

The control terminal, or selection point, will generally be, but need not necessarily be, geographically separated rorn the location o receivers i2 and noise detectors 2B. The unidirectional merit indicating signal from each noise detector 2t is received by a respective longitudinal balancing unit 2li, 2li', etc. at the control terminal. In this unit, the signal is converted from a balanced-to-grcund signal to a singleended or unbalanced signal, and any longitudinal noise currents which may have been added to it in the connecting path 22 are effectively eliminated. The signal output of each longitudinal balancing unit 2li, 24', etc. consists of a unidirectional voltage, vvhich has at any instant an a-mplitude that is representative of the weighted amount of noise energy that accompanies the audio frequency message signal energy in the output circuit of a respective receiver. Since, as was previously stated, the message signal energy in the output of each receiver is at any instant at the same level, it follows that a comparison of the noise energy indications, or merit indicating signals, for the receivers is in eiect a comparison of the signal-to-noise ratio of the respective receivers. This comparative function is performed in receiver selector I6 at the control terminal. For general descriptive purposes, this unit is functionally indicated in Fig. l as a mechanical switch arrangement comprising the main contacter 2B and switches 28, 28', 28", etc. In addition, a threshold circuit 3! and threshold switch 32 are functionally shown as entities separate from the receiver selector I6. Actually, as will be noted in connection with the description of Fig. 4, these latter units may form a portion a-n appreciable effect on the audio 'acconti of the receiver selector I6. Selector I6 is arranged such that it responds to the merit indicating unidirectional voltage of greatest amplitude as received from the various longitudinal balancing units 2t?, etc. to actuate its contactor 2t and close the circuit of only one switch 2B, 2S', etc. at any instant. The closure of a switch 28, etc. connects to the output circuit 34 the audio signal output of the radio receiver that produces the unidirectional merit indicating signal of greatest amplitude. Concurrently, with this action, the merit indicating signal from each balancing unit 24, 2t', etc. is supplied to the threshold circuit 30, in which it is determinedy whether any one of the signals exceeds a predetermined minimum value that has been chosen as being representative of just usable or minimum quality received signals. If any one of the merit indicating signals exceeds this minimum value, it is indicative that some one of the receivers l2 is producing in its output circuit, the desired message signals at a level which, with respect to any accompanying noise currents or signals, has been previously adjudged to be suitable for service purposes. Under these conditions, the normally non-operated threshold switch 32 is closed, and output circuit 34 is connected to utilization circuit i8. Utilization circuit it may be any suitable terminating circuit such as a subscriber subset, a connecting circuit such as a trunk; circuit, or a switchboard termination. If none of the merit indicating signals exceeds this minimum, or just usable value, the threshold switch 32 remains non-operated; notwithstanding that receiver selector IG may have actuated its contacter to select the best one of the subnormal indicating signals which is received' from balancing units 2li, 2t', etc., and, in so doing, has connected the signal output circuit of the appropriate receiver to the selector output circuit 3d.

The manner in which these described functions are performed may best be understood from a consideration of Figs. 2, 3, and 4 when arranged in the fashion shown in Fig. 6. In Figs. 2 and 3, only one noise detector 2) and one longitudinal balancing unit 2t are shown, inasmuch as it is believed that an understanding of this portion of the practice of the invention can readily be secured by a consideration of one selecting .i

branch only. It should, of course, be appreciated that a complete selecting system would ordinarily employ as many detectors 20 and balancing circuits 2e as there are receivers l from which a selection is to be made. In Fig. 4, the details of two, the upper and lower, selecting channels only are shown, since it is believed that a thorough understanding of this portion of the invention may be secured from a consideration of the details and action of these two channels. Itis to be understood that the circuit details'of the intermediate selecting channels, the presence of which is diagrannnatically indicated by the block representations, are substantially the same "as those shown for the other channels'.

Detailed description Receiver E 2 is shown at the upper left of Fig. 2'. The output of this receiver is unbalanced with respect to ground, and is connected through a resistance pad or attenuator 2e and a transformer 31 to conductor pair 33. The attenuator 2Q is provided so that the impedance of the line, as seen throughv the transformer 3l, will not have frequencyand noise voltages appearing on conductor or lead 36. At this point, it may be noted that conductor pair 33 forms the same conductive connections that were shown separately in Fig. l as connections i and 22. One portion of the signal output from receiver l2, constituting the message channel thereof, is transmitted over conductor pair 33 to the receiver selector unit it' (Fig. 4). A second portion of the output from this receiver, constituting the merit indicating signal channel thereof, is transmitted to noise detector over conductor 3&5 to a frequency sensitive network comprising resistors 40, 42, and d and capacitor 3B. This network discriminates in favor of the higher frequency noise energy, and aids in evaluating the interfering effect of unwanted interfering periodic radiations which may be modulated by low frequency components. One portion of the output from this network is connected over conductor 86 to a band-pass filter at, the values of the circuit elements of which are so chosen that is rejected all frequencies within the band of message signal frequencies, and passes only frequency components both above and below this signal hand. It is a purpose of this lter to eliminate, in so far as it is practicable, the message signal frequencies that are supplied over conductor 45. In a tested embodiment of the invention, this filter was designed tc reject frequencies in the band between 200 and '7000 cycles per second and to pass or transmit frequencies outside of that band. Noise or interference frequency currents that are passed by lter :it are connected through transformer 5t to the input of a variable loss circuit, which comprises transformers 5S, Sil, unidirectional conducting devices 52, and capacitors t, 58. Devices 52, 56 may be constructed of any of several suitable materials, and each has the characteristic that it conducts current in one direction with considerably greater facility than it does in the opposite direction, and also that its resistance in its conductive direction' is controlled to a marked e3-:tent by the magnitude of the potential existing across the unit. Devices having these general properties are often termed varistors, and will he so designated in this description. In a testedl embodiment of the invention, these devices 52, 55 were dry-disc copper oxide type of varistors. Capacitors 58, 54 act to neutralize the capacities that are inherent in this type of varistor, and together with these inherent capacities form a balanced capacitive bridge arrangement. The values of capacitors 5S, 5t will, of course, depend upon the inherent capacitance of the actual varistors that may be used. Conductors 62, te are connected tothe mid-points or" the windings of transformers 5t, 6d and serve to impress across the varistors 52, 55, a varying potential which is derived in a manner which will now be described.

Capacitors el, 92, 33 and inductor Sil comprise a band-pass lter which has minimum attenuation at a frequency corresponding to the middle of the message signal band. The attenuation of this nlter at frequencies outside the band ofr rnessage signal frequencies is relatively high. The message signal frequencies are passed through a transformer 94 to a full wave rectier 95 to develop across potentiometer tit and f lter capacitor 96 a unidirectional voltage which is indicative of the presence of and the strength ci message signal frequencies in the output of the radio receiver. The polarity of this rectied voltage is opposite to the polarity of the voltage that is developed across potentiometer 84 bythe anode- 7 cathode current of the electron discharge device, specifically a pentode, 8B, which also flows through this potentiometer. Conductors 62, B4 are connected across potentiometer 84 in such fashion that they apply to the varistors 52, 5E a potential which is the algebraic sum of the potentials developed across potentiometer 84 as described. It might be stated at this point that this last described circuit branch, comprising the full Wave rectier 95, is provided in order to desensitize the noise detector 2f.) during periods when the level of the message signals at the transmitting station is exceedingly high. The reason for this will be developed in connection with the description ci the operation of the circuit. Thus, the transmission loss through the variable loss circuit is controlled by the potential that is applied over conductors 62, E4 to the varistors 52, 56 in this circuit.

The secondary winding oi transformer .50 is terminated by potentiometer 59. The control grid of an electron discharge device, speciiically, a pentode 6| is connected to the movable arm of potentiometer 59, and this device comprises the iirst stage of a two-stage ampliiier which includes the pentodes El, 66. This amplier may be in accordance with conventional design. Its circuit constants are so chosen that it has a substantially uniform ampliiication characteristic over a range which includes any desired frequencies above and below the message signal frequencies. The output of this amplifier is connected through a transformer 68 to a full wave rectiner circuit comprising a double diode 12, terminating resistors l0, 'il and load resistor 1E. rlFhe output of this rectifier is passed through an energy storage and lter arrangement comprising load resistor l5, a capacitor 14, and the resistor-capacitor combination 13, T1. The time constants of the rectifier and storage circuits are chosen such that a finite time is required for the energy contained in sudden sharp bursts of noise to develop full potential across capacitor l. In one tested embodiment, a suitable time constant for this circuit was found to be about 100 microseconds. After a potential has been developed across capacitor M by a surge of energy, this condenser will discharge partly through resistor 'l5 and partly through the resistor-capacitor combination i6, il. This latter combination forms a filter circuit which prevents undesirably sudden changes of potential from accumulating across capacitor il. The discharge time constant of capacitor 'i4 is preferably relatively long with respect to its changing time constant, and in one tested embodiment of the invention a discharge time of about l0 milliseconds was found to be satisfactory. The successful practice oi the invention is, of course, not limited to the use ci the above-mentioned time constants. Any other suitable time constants may be used with equal facility. ln general, it is believed that the relationship of the charging and discharging rates should be correlated with limiting factors in other portions of the communication system; for example, such factors as the cut-ofi characteristics oi the signal channel, and the response of, or appreciation period, of a limiting unit in the utilization process. In an aural communication system, wherein the highest frequency oi the message signal is restricted to some definite value, and in which some element, such as the ear, has a fairly denite minimum appreciation period; it might be desirable to restrict the charging period to a value suitably less than one-half the cyclic period of the highest usable signal requency, and to arrange the discharge period at a value suitably less than the minimum appreciation period of the ear. Such an arrangement imparts to the detected interference components a weighting factor that is variable with frequency, and which causes the control eiiect of these detected components to vary in a manner that is similar to that in which the impairment eiect of noise components is observed to vary in the utilization circuit. At the same time, the response of the detection device is fast enough to permit the proper evaluation, or appreciation, of unde sirable quantities of interference energy in a shorter period than is required by the limiting receptive member for a similar appraisal. The control electrode of pentcde Se is connected to the common point of resistor-capacitor combination lli, l?. The cathode circuit of pentode 8i? includes an adjustable resistor il?. and the potentiometer across which are connected conductors 52, 54. Anode power supply for pentode Pi is secured from the respective longitudinal balancing circuit 2t (Fig. 3) at the control terminal point or site. This potential is transmitted to the location of noise detector 2li over the upper conductor or" conductor pair 33. The lower conductor oi conductor pair 33 forms the return path for this circuit and is connected to the anode-cathode circuit of pentode 8i] at the lower end of cathode load potentiometer 343. Capacitor joins the upper and lower sections of the secondary Winding of transformer 3l to form a conductive path for the voice frequency signal currents from receiver l2, and to block the direct current that is iioviing in the upper and lower conductors oi pair '53.

A similar blocking function is performed by capacitor lill at the longitudinal balancing unit end ci conductor pair 33, as is shown in Fig..3. Transformer lilo is inserted in the circuit of conductor pair 33, and together with capacitor IDI, forms a blocking circuit for the direct current derived from potential source i213 through resistor m2. The anode-cathode circuit of pentcde 80 (Fig. 2) inclu es potential source iil, load resistor lili?, the upper conductor ci conductor pair 33, resistors S2, Sli, the lower conductor of conductor pair 33, resistor lil', a portion of `Voltage dividing potentiometer los to the grounded terminal or" source 26.

Voltage dividing potentiometer ltd provides a convenient and ready means of adjusting the potential of the cathode or pentode S0. This potentiometer provides a means for imparting a small fixed adjustment to the unidirectional signal voltage as it is obtained from the noise detector 2d, and permits the proper coordination of these voltages from the various noise detector units of a selecting system. Resistors m2 and 63 may be approximately equal and of any suitable value consistent with the length of conductor pair 33 and the characteristics of pentode 80.

With respect now to the longitudinal currents balancing circuit or unit 215i, anode potential for balancing electron discharge device or pentode IBS is supplied from potential source l2 through anode load resistor m5. Resistors H35, 108, potentiometer it? and anode load resistor |55 form a voltage dividing circuit. The movable arm of potentiometer ll i connected through a coupling capacitor H5 to the control electrode of pentode 4GB. Values for these components and the dynamic characteristic of pentode H39 are coordinated such that, the amount of voltage change that is applied to the control grid of pentode i09 is just sutcient to counteract in its anodecircuit the voltagechange that occurs at the .junction of resistors |02, |03. For example, if the potential at the junction of these latter resistors changes by one volt in a positive direction, the potential at the anode of pentode |69 Will cha-nge by an equal amount in the opposite direction. Because of the large amount of voltage feedback present in the anode control grid coupling circuit, the transmission equivalent between the junction of resistors |62, |38 and the anode of pentode |69 is determined primarilyv by the voltage dividing circuit comprising resistors |66, |91 and IGS, and is relatively independent of changes in the other parts of thecircuit. Resistors ||6, I I'I, H3 form a voltage dividing network for the proper biasing ci the control electrode of pentode |519. The anode circuit of pentode It is connected through coupling capacitor Il@ to two volta ge ydividingcircuits, one of which includes equal valued resistors and I I2, the other of which includes equal valued resistors H3, Ils. The value of these resistors may, but need not be, the same so long as the paired resistors are equal valued. Conductors. |22 and |24, are connected at one end of each to the junctions of resistors ill, ||2 and H3, Hd respectively. The other ends of these conductors are connected to the input cir-cuits of the selecting and threshold branches, respectively, of receiver selector' IS (Fig. 4).

The noise indicating signal on conductor |22 is supplied to the input of the channel selection branch of selector I6 (Fig. 4) which branch includes an electron discharge device, specically, a triode |33. Resistors |23 or |28 and capacitor |32 constitute an integrating network for adding a predetermined vdelay to the voltage changes that are supplied to the control electrode of the triode ISG. Whether resistor |26 or resistor |28 forms a component of this integrating network depends upon whether relay 68 is operated. Resistor |28 is smaller than resistor |23 by any desirable amount in order to provide a faster selection by triode |34 at the beginning of a transmission interval.

In its non-conduction state, the anode-cathode circuit of triode |315 includes source |33 of anode potential, conductor |37, the winding of relay |33, the space-discharge path of triode |34, conductor |40, Contact i of relay |42, conductor Uitl, cathode-biasing resistor it@ and a source |48 of negative potential, the positive terminal oi which is connected through a common ground connection tothe negative terminal of source |36. A source il@ of negative potential is connected over conductor to the winding of relay M2, and meets operating ground over the contact of relay |38 when this latter relay is operated. Resistors lil, &2 are connected over conductors il, |45 to form a voltage divider across a biasing source |54 and its iilter capacitor |55. The cathode oi triode 531i is connected to the junction of these resistors. When relay $42 is operated, in a manner which will be presently described, the message signal conductor pair 3S is connected over contacts 2 and 3 or" this relay and conductor pair ifi-3 to contacts 5 and 3 of relay |3 in the threshold branch of the circuit. Operation oi this er relay connects the signal output of the selected receiver I2 (Fig. 2) to the utilization circuit i3. The operation of relay |58 also supplies ground over contact l and lead |23 .to a signal lamp in the terminating switchboard of utilization circuit i3, when provided, to indi- 1,0- cate that some one of the receivers |2 is producing a message signal of usable quality.

The unidirectional voltage signal from each longitudinal balancing unit 24 is supplied over the respective conductor |24, |24', etc. to a unidirectionaldevice |36, |56', etc., in the threshold branch oi` the circuit. The cathodes of these devices are connected in parallel. and their circuit includes load resistors |58, |60 and a portion of potentiometer |62. The control electrode of an electron discharge ,device or triode |64 is connected to the common point of resistors |58, |60. The lower end of resistor E!) is connected to the movable contact of potentiometer |62, which forms a voltage divider across the source |48 of negative potential. The anode-cathode circuit of triode |65 includes the source |35 of positive potential, the winding of a relay |33, the space!l discharge path oftriode |64 and a ground connection which is common to the source |36. The operation of relay 65 connects operating ground over its Acontact to one terminal of the winding of relay |33, the other terminal of which is connected to the source of negati-ve potential.

YIt will be noted from Fig. 4. of the drawing that, in this described embodiment of the invention, selector I3 comprises four parallel selecting branches, or channels. The circuit details of each of these channels are substantially identical with those of the rst, or upper, channel which have been described. Like circuit elements have been assigned the saine reference numeral irl-each channel, and either a single, or double, or a triple prime exponent has been added to the numeral to properly identify the element in its branch, or channel location. Most of the circuit details of the intermediate branches, or channels, of selector l5 have been .omitted in the interest of simplicity, and the presence of these branches is indicated by the block diagrammatic representations.

Descrzptzon of operation rThe operation of the selecting system of this invention, the structure or circuit arrangement of which has just been described, can best be understood from the following description of the operation of the arrangement shown in Figs. 2, 3 and 4. Assume rst that no receiver of the system, as exeinpliiied by receiver |2 (Fig. 2), is receiving a usable train of message signal Waves. Under these circumstances, the noise energy in the output circuit of receiver 2 Wil-l ordinarily be at a relatively high level. In a manner which will be presently explained, the bias voltage on the control electrode of pentode s@ in noise detector 29 will be at a relatively high negative value, and the current flowing in 'its anode-cathode path and in conductor pair 33 will be low. The voltage drop across resistor |63 (Fig. 3) will be small, and the potential' that is supplied to conductors 22, |24 by the potentiometer circuits comprising resistors ||2, ||3, H4 will be low. The actual value of this potential on conductor I 22 may or may not be suiiicient to cause triode |34 to conduct, and in so doing to operate its anode load relay |38. If relay |33 is operated, it completes the ground `connection for operation of relay |42, thereby connecting the signal output of receiver i2, on conductor .pair 33, over contacts 2, 3 of relay |42, to interconnecting pair |43, the circuit of which is open at contacts 5 and 6 of relay |68. The potential supplied over conductor |24 to diode |55 in the threshold branch of selector IE will be low, and the voltage that is developed across voltage divider |58, |60 will also be low. The portion of this potential that is supplied to the control grid of triode |64 will be insuiiicient to overcome the negative bias that is derived by its control electrode from potentiometer |62. Under these circumstances, triode |613 will not conduct current, its anode load relay |55 will not be operated, threshold relay 68 will not be connected to ground from the contact ci relay |65 and will remain unoperated, and the circuit from conductor pair |433 to the utilization circuit I will be broken or open at contacts and (i oi r..- lay |68. Therefore, notwithstanding that the upper selecting branch of selector I6 may have functioned to connect the signal output of receiver i2 to interconnecting pair M3, the output of this receiver is not connected to utilization circuit I3 because the threshold circuit has adjudged the received noise signal to be representative of output signals that are less than the just usable, or minimum, commercial grade. This just-usable or minimum value signal is a predetermined quantity, and may be changed by varying the position of the movable arm of potentiometer |52. grade of audio message signal exists in the output of receiver I2. Because the level of energy in the output circuit of the receiver is controlled by the level of modulation at the transmitter, the noise energy in the output circuit of the receiver will be reduced from its former high value by some unspecied amount. This revised output energyis treated in the same manner as was the previous noise energy. A portion of it is transmitted through attenuator 29 and transformer 3| to conductor pair 33, and over this conductor pair to contacts 2 and 3 of relay |42 in receiver selector I6. Assume for purposes of this explanation that, at this time, this relay is unoperated and contacts 2 and 3 are open. A second portion of the signal output from receiver i2 is supplied over conductor to the frequency sensitive network comprising capacitor 3S and resistor 40 and voltage dividing resistors 42, 44. The transmission loss through this network varies inversely with frequency. The network has two output circuits, one of which is connected to the band-pass lter comprising inductor 9|) and capacitors 9E, 92. The message-signal output from this lter is supplied through transformer 94 to full-Wave rectier 95. The rectiiied output voltage is applied over conductors |527 54 to the variable loss circuit comprising varistors 52, 56, and tends to control the loss through this circuit by controlling the impedances of the varistors. The second output "I from. the frequency sensitive network` 38, Ail is applied over conductor to the input of bandreject filter 48, the characteristics of which are such that it passes frequencies both above and below, but not within the band of message signal frequencies. Because of the frequency discriminating action of network 38, 4Q, the higher frequency noise components are transmitted to band-reject lter Q3 with greater facility than are the low frequency components. As Was previously stated this arrangement is helpful in evaluating the interiering efiect of unwanted radiations which may be modulated oy low irequency components. The noise energy that is passed by iilter iS is transmitted through the variable loss circuit comprising transformers 50, and varistors 52, 56 to the amplier comprising devices ti, S5. This amplified noise energy is rectified in the full-wave rectifier circuit comprising the double diode 12, balancing resistors Now assume that a usable 10, 1| and load resistor 15, together with its filter which includes capacitors 14, 11 and resistor 16. Because of the limiting action in receiver I2, less noise energy will now be rectified than was previously the case, and the potential across capacitor i1 will he reduced. The negative potential at the control grid electrode of pentode will now be less than in our previously assumed case, Where no message signal was being received. The anode-cathode current in this pentode will now be increased, and will provide a greater voltage across potentiometer Bt than heretofore existed. The potentials that are generated across this potentiometer by the passage through it of cathode current from pentode 8B and the rectified current from rectifier Q5 are opposed in polarity. The net potential is supplied over conductors S2, Si to the previously mentioned variable loss circuit comprising varistors 52, 56 Where it controls the transmission loss through this circuit by affecting the characteristics of these varistors.

The action of this feedback arrangement is to lineariae the relationship between the current flowing in the anode-cathode circuit of pentode 89, with respect to logarithmic variations in the level of the noise input to detector 2B. This action comes about because the magnitude of the anode-cathode current of pentode 36 is an inverse function of the level oi the rectied noise energy. As the noise level is decreased, the cathode current of pentode 89 and the potential across potentiometer S4 are increased. This increased voltage is fed back over conductors 62, 64 to the variable loss circuit where it decreases the transmission loss by controlling the characteristics of varistors 52, 56. This action tends to increase the rectied noise energy, and thereby reduce the cathode current of pentode si), and the potential across potentiometer Sli. The n et effect of this arrangement is to produce a stabilized system having the desired characteristic, such as is shown by the idealized curve |12 of Fig. 5. Curve |12 shows the manner in which the anode-cathode current in pentode 8B may be changed by logarithmic variations in the level of the noise energy that is applied to the input of detector 20 when sufficient voltage is fed back to control varistors 52, 55 possessing suitable current-voltage characteristics. Ii no feedback arrangement, or its equivalent, were used the input-output characteristic of detector 28 would be similar to that shown by curve H of this figure. Tt will be noted that this latter curve varies considerably from the more desirable logarithmic characteristic and that its operation is restricted to a relatively small range of input levels. The departure may be great enough that signals having a suitable or acceptable noise ratio would be rejected. From the described action of detector 2B, it will be appreciated that this unit produces in its anode-cathode circuit, and, therefore, in the direct-current circuit which includes conductor pair 3S, a current the magnitude of which increases linearly as the logarithmic relation between the signal and noise in the output of receiver I2 is increased. Therefore, for low noise values this current will be at its maximum values. Potentiometer Sri is made adjustable to permit control oi the amount of this fed back voltage so that essentially the same input-output characteristic may be obtained from each noise detector 29 in the selecting system.

Because non-linearity generally exists in the transmission path between the input to a radiotransmitter and the signal output of its asso- 13 ciated receiver, there results a distortion product or products whenever a message signal is transmitted. This distortion product may not be appreciable under ordinary circumstances, but it becomes more prominent as the input signal attains an extraordinary high level. rl'his corresponds to a highlevel of modulation, which produces a loud or high level output signal. Many of these distortion products fall in the frequency range above and below the signal frequency band, and are thus passed by the bandereject filter 8. Since they are passed by this lter, the detector circuit 2t would normally treat them as indications of objectionable noise conditions, if it were not for the operation of that portion of the circuit that comprises full-wave rectier S5. This portion of the circuit receives some of the audio signal energy from the frequency sensitive network 38, ill! and produces from it a unidirectional voltage, the magnitude of which is indicative of the level of the message signal, and the polarity of which is opposite to the polarity of the voltage generated across cathode potentiometer 84 by the flow of cathode current in pentode Sii. Being thus poled, this rectified voltage acts on varistors 52, 56 during period of high message signals to increase the transmission loss through these units, and thus to momentarily incapacitate noise detector Eil. IThrough this action, the noise detector Ell is prevented from mistaking distortion products, which arise from otherwise desirable message signals, as objectionable interfering noise currents.

It will be noted that detector 2i? is isolated from direct ground connection, and that the anode power supply for pentode iii is derived over the line 33 from the associated longitudinal balancing unit 24 (Fig. 3) which may be, and probably will be located at a considerable distance from detector ZG. None of the circuit elements associated with the line and pentode Si". have any direct current connection to the ground at the receiver location, the potential of all points in that part of the circuit being determined with respect to the ground potential at the control terminal. This prevents a diiierence in ground potential between any receiver station and the control terminal from altering and thus falsifying the indication signal.

The anode-cathodc current of pentode B, the magnitude of which is indicative of the ratio of the signal and noise in the output circuit of receiver 2, ilows through conductor pair 33 and, therefore, is subject to any induced interfering voltages. Since this circuit is isolated from ground potentiai, these interfering voltages will be induced in equal amounts of the saine polarity in each of the conductors of the pair.

The action of the iongitudinal balancing unit 24 is such that the effect of these induced voltages is eliminated. Pc ive potential is supplied from source i253, through anode load resistor |02, over oneconductor or" pair 33, to the anode of pentode at in detector l (Fig. 2). rihis tubes cathode current flows through potentiometer 8d, one conductor of pair 33, through resistor |83 and biasing resistor its to ground. Therefore, as the anode-cathode current through pentode Bil Y(Fig. 2) increases, the potential with respect to ground at the lower end of resistor |22 is decreased and at the upper end of cathode resistor |03 is increased. These resistors are substantially equal in value and, therefore, these changes CII potential are equal land opposite. -A portion 'of the potential change across anode resistor |02 is applied through the movable contact arm of potentiometer le? and coupling capacitor H5 to the control electrode of pentode |09. This vol*- age produces vin the anode circuit of pentode |09 a voltage change which is equal in amplitude and opposite in polarity to the change across resistor |22. This equal and opposite change is coupled through coupling capacitor H to the parallel voltage dividing circuits comprising resistors l, ||2 and H3, llt. Since the voltage at each end ci the dividers is changed by the same amount, the potential of conductors |22, |24 with respect to ground is changed by an equal amount. Ii, however, interference voltages of the longitudinal type are induced on conducting pair 33, there will .appear at the lower and upper terminals of resistors H12, |33, respectively, potential changes which are equal and of the same polarity. Under these circumstances, when a portion of the change that appears at the lower terminal or" resistor 262 is applied through the movable contact of potentiometer lo? and coupling capacitor H5 t0 the control electrode of pentode |059, there appears in the anode circuit of this pentode a potential change which is equal in value but opposite in polarity to the potential change at the upper end of resistor |53. Since equal but opposed changes are made at the two ends of the voltage dividers, the potential on connecting circuits |22. i261 remains unchanged, and the inter- Jfel-ing' effect of the induced longitudinal voltage is eliminated.

The voltage on conductor |24 controls the operation of the threshold branch of selector I6 (Fig. 4) while that supplied over conductor |22 controls the operation of the receiver switching or selecting branch of the selector unit. In the following explanation of the operation oi receiver selector i6 it will be understood that, although oni-y one longitudinal balancing unit 2d (Fig. 3) is shown as providing unidirectional voltage signals to conductors |22, |255 of selector IS, similar signals are supplied over conductors |22', |24', etc. to the remaining switching, or selecting, and threshold branches of this circuit. These voltage signals are obtained from the noise detectors 28', 29, 2Q and longitudinal balancing units 24', 24", 24' that are associated with the other receivers of the communication system. It will also be understood that the details of these circuits, although they are not sho-wn on Figs. 2 and 3, are to be taken. to be substantially the same as the details there shown for that one selection branch. The voltage on conductors |22, |24 is, as was stated, a single-ended signal, the magnitude of which varies inversely as the level of the noise energy varies in the output of the respective receiver. in our assumed example, the signal in the output circuit of receiver i2 is changed from one which consisted almost entirely of noise energy to one in which the message signal energy is dominant, and a relatively small amount of noise energy is included. The voltage signal on conductors |22, |211 is accordingly changed from a low voltage value to a relatively high voltage value. As the voltage on conductor |2i is increased, the negative bias on the control electrode of threshold tube 55 is counteracted by the potential developed across the rectifier load comprising resistors |53, itil, and the potential of this electrode is raised to a value where the resulting anode current now is suicient to operate relay iet.. This vcondition corresponds to the reception of a. usable orcommerciai grade of signal by re- Vcei'ver t2. It will .be readily seenthat this same action would oo cur if this commercially usable signal had been received by any one of the other receivers l2', l2 or ll". Relay |66 connects ground over its Contact, through the winding of threshold relay Ii, to potential source llo and causes this latter relay to operate, which action opens contacts i, 2, 3 and 4 and closes the contacts and G of this relay. The closure of contacts 5 and E connects the output of a selected receiver, in this assumed case receiver l2, to the utilization circuit i8.

The manner in which receiver |2 is selected and the remainder of the receivers are rejected by the selection branch of selector I6 will now be explained. It was assumed that the anode current flowing in triode |34 was insucient to hold its anode load relay |35 in the operated condition. When the voltage signal on conductor |22 changes from a relatively low value to a rela tively high value, which value is in excess of any voltage signal that exists on incoming conductors |22', i22, iiw', triode Kili is caused to increase its anode-cathode current to a value that is sufficient to operate relay |38. Relay |38 connects ground over its contact to the winding of relay ifi. The operation of relay M2 opens the short-circuiting connection across resistor |52, provided by normally closed contact of relay |42, and closes over its contacts 2 and 3 the connection from conductor pair 33 to interconnecting pair |43. Since pair |153 is closed through contacts 5 and 5 of relay its to the utilization circuit i2, the output of receiver l2 is connected to this latter circuit.

t will be noted that resistor |46 is included in the cathode circuit of each triode |34, i3d', etc. The voltage that is developed across this resistor by the combinedl current flow in all triodes i3d, its', etc. exerts a degenerative feedback effect which tends to bias the cathode of each tube positive with respect to the potential of its control electrode. Therefore, if the same potential exists on each control grid electrode, the cathode of each triode will be at the same potential, however, if the potential of the control electrode of one triode exceeds that of the control electrode of any other triode, the current flow through the triode having the highest potential on its control electrede will establish the potential at all of the cathode electrodes. Therefore, as the current flow in triode l increases because of the increased potential on its control electrode, its cathode current flowing through the common load resistor i?? increases the potential of its own cathode electrode and that of all other connected cathode electrodes. If the potential increase is suiciently large the entire anode-cathode current flow will be transferred to triode |34, and the remainder of the trio-des 634i', |34", etc. Will cease conduction. This condition will continue until some one of the longitudinal balancing units Eri', 2t, etc. supplies to its associated selection branch of selector l a potential which is substantially equal to that existing on the control grid electrode or triode |36 in the upper branch. lf such a condition prevailed receiver selector I6 might connect the output of tivo receivers, for example, receivers i2 and l2 to the utilization circuit l, or it might be subjected to undesirable indecision by switching from the output of one to that of another of the receivers as the noise energy in the ouput circuits of these receivers fluctuates one above the other by a slight amount. To prevent these conditions, a preferential biasing arrangement is provided such that when a usable vhined resistors |59, 52 a. voltage signal has been chosen by selector IB, it will continue to be chosen until it is supplanted by a more desirable signal which exceeds the chosen signal by a predetermined amount. This preferential biasing feature is brought into action when relay |42 opens the circuit which was formerly closed over its contact This action removes the short-circuit connection which formerly existed across resistor |52, and produces across the comdrop which is equal to the potential of source 151i. The effect of this action is to lower the potential of the cathode electrode of triode |34 With respect to the potential of the remaining cathode electrodes by a predetermined amount which is controlled by the relative values of resistors |55, |52 and the potential of source |54. Under these circumstances then, beore any other branch of selector lli can connect the output of its associated receiver to utilization circuit i8, and in so doing bias triode ist of the upper branch to its non-conduction state, the potential on the control grid electrode of its associated discharge device must exceed the potential on the control grid electrode of triode |34 by at least the amount of the voltage drop across resistor |52.

Each electron discharge device or vacuum tube |34, |34', etc., of the selector tube array is preferably chosen for a high transconductance Wherein a relatively small change in grid-cathode potential diiference Will produce a relatively large change in anode-cathode circuit current. As already referred to hereinabove, the circuits associated with the electrodes of these tubes include feedback by virtue of the common cathode resistor ll, to establish an anode-cathode current in that one of the tube array whose control grid has received the highest-valued input signal, that is, input signal respective the best merit-indieating signal. This anode-cathode current remains substantially constant even though the potential applied to the control grid of such one tube varies or changes over a range corresponding to a Wide variation or input; assuming, of course,

' that the range of variation of the potential applied to the control grids of the other tubes during such period is at all times less than the then applied potential on the control grid of the one tube, taking into consideration, also, the preferential bias provided by the resistor |52, |52', etc. However, when the control grid potential on one of the other tubes is such that current is caused to flow in the anode-cathode circuit of such second tube, then a small variation in the potential of either such one tube or such second tube produces large changes of current in their respective anodecathode circuits. ln a tested embodiment of such a tube array, the devices |34, |34', etc. Were R. C. A. type GSLYGT; the resistor Was one of 100,000 ohms; resistors ld, |50', etc., and resistors m2, |52', etc., were of 1000 ohms each; the relays |38, |33', etc., were chosen and adjusted for operation on approximately .7 milliampere and for release on approximately .4 milliampere; cathode-anode circuit current through the cathode resistor |55 varied between approximately 1.5 and 1.9 milliampere.

For descriptive purposes, let it now be assumed that there is received on conductors |22" and |24'", a signal having an amplitude that is greater than that of the signal on conductors |22, |24 by more than the voltage drop across resistor |52. The main anode-cathode conduction path will be transferred from the upper selection branch. whlchincludes triode |34, to the lower selection branch, which includes triode |34'", because of the degenerative bias that the increased anodecathode current in triode |34 imposes upon the control electrode of triode |34. Under these circumstances, triode i134 will cease conducting, relays |39 and |42 will release, and, in so doing, open the connection between conductor pairs 33 and |43, and restore the short-circuiting connection across resistor |52. Simultaneously with this action, relays 38W and |42'" will be operated to connect conductor pair 33 to conductor pair |43, and to open the short-circuiting connection around resistor |52" over contact i of relay id'", to add the preferential bias voltage across resistor |52" in favor of the lower selecting branch. The lower selecting branch will continue to connect the output of its associated receiver |2 to the utilization circuit i8 as long as the merit indicating signal provided by its longitudinal balancing unit Zt" continues to exceed that supplied by any of the other associated balancing units. Since the potential that existed at the control electrode of triode |64 in the threshold branch of selector l5 was adequate to provide sufncient anode-cathode current in this triode to hold operated the anode load relay I 65, the increased signal on conductor 24"' causes no added eiect.

If the voltage that is derived by the threshold circuit from conductors |24, time decreased to a value where the potential on the control electrode of tube 64 is insuiiicient to hold relay |65 operated, the connection between conductor pair |43 and utilization circuit 8 will be broken; notwithstanding that some one of the selecting branches of receiver selector l5 may be operating to connect the output of its associated receiver to conductor pair M3, and to exclude the output of all other receivers in the system. If this condition exists for an undesired interval, and it is desired to accept any one of whatever understandable or decipherable receiver outputs are actually present, appropriate adjustment at the control terminal of the potentiometer |52 will effect a shift downwardly of the operating level of the device |64, and render what would not be acceptable with the initial setting of the potentiometer, connectable through to the utilization circuit. Of course, if instant radio transmission conditions or the presence of extraneous industrial interference should preclude any character of usable or understandable signal being produced in the receiver output circuits, use of the system necessarily would await upon the clearing of such conditions.

The invention has been described as being incorporated in a receiver selecting system that is arranged to choose between the output signals of four receivers of the saine message signal wave. It should be evident that this is only one embodiment of the invention, and that variations of this arrangement which do not depart from the spirit and scope of the invention will occur to those skilled in the art. Although the invention has been described as being incorporated in a radio communication system, it should be appreciated that it may be applied with equal facility in a variety of different types of selecting and switching arrangements in which the relative desirability of the various units or circuits is indicated by an appropriate derived or representative electrical factor, for example, a unidirectional voltage signal.

What is claimed is:

1. In combination, a plurality of control signal i255', etc. is at any paths, which control signals may be respectively of diierent control merit in a given interval and respectively variable in control merit during sucn cessive intervals, utilization means to be controlled during any interval by that one of the control signals incoming over said paths that is then of the better control merit, a like plurality of electron discharge devices each including a cathode, anode and control electrode, cathodeanode and cathode-control electrode circuits for each of said devices, a source of cathode-anode circuit current for said devices, a load resistor common to the cathode-anode circuits oi said devices for biasing substantially to cut ofi all of said devices except that one whose control eiecu trode has impressed thereon the control signal oi better control merit, means coupling a control signal path to the cathode-control electrode circuit of a. respective one of said devices, and means in each of said cathode-anode circuits to effect during any interval a desired control with respect to said utilization means, upon cathode-anode circuit current-conduction of preassigned degree in one of said devices responsive to that one ci the control signals of the better control merit.

2. The invention of the preceding claim in which 'the cathode-anode circuit of each of said devices includes means for applying a preferential bias to the device when the device is the non-cutoff one of said devices, whereby said non-cut-of device may in turn be cut off only by the impressing on the control electrode of another oi said devices, of a control signal of better control merit by a predetermined margin over that applied to such non-cut-oi device.

3. In combination, a plurality of electron dis charge devices each comprising a cathode, an anode and a control electrode, cathode-anode and cathode-control electrode circuits for each of said devices, a cathode-anode circuit current source for said devices having a positive potential terminal connected to the anodes of said devices, a load resistor common to the cathode-anode circuits of said devices and having one terminal thereof connected in common to the catho-des of said devices and having another terminal thereof connected to the negative potential terminal oi a second current source, a signal input circuit respective to each cathode-control electrode circuit, said input circuit including voltage integrating means for imparting a time delay to the signal variations applied to said respective cathoden control electrode circuit, and an output circuit respective to each of said cathode-anode circuits.

4. The invention of claim 3 in which said voltage integrating means may be varied from one to another value thereby imparting a greater or lesser time delay to the signal variations applied to said cathode-control electrode circuit, and means for controlling said integrating means in accordance with the applied signal.

References Cited in the iile of this patent UNITED STATES PA'IENTS Number Name Date 1,593,993 Sprague July 27, 1926 1,801,657 Buyko Apr. 21, 1931 1,819,599 Francis Aug. 18, 1931 2,350,888 Hall June 6, 1944 2,535,377 Titterton Dec. 26, 1950 2,552,013 Orpin May 8, 1951 2,611,017 Bailey Sept. 16, 1952 2,632,046 Goldberg Mar. 17, 1953 2,637,810 Moerman May 5, 1953 

